1. Field of the Invention
This invention relates to a method for adjusting positions of a plurality of radiation images, which are to be subjected to superposition processing or subtraction processing, by eliminating shifts in positions among the radiation images. This invention particularly relates to a method for adjusting positions of radiation images, wherein image patterns of a marker for position adjustment need not be embedded in the radiation images.
2. Description of the Prior Art
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation, which carries image information of an object, such as a human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, when it is exposed to the stimulating rays, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed, and the processed image signal is then used during the reproduction of a visible image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The visible image finally obtained may be reproduced in the form of a hard copy or may be displayed on a display device, such as a cathode ray tube (CRT) display device.
Techniques for carrying out superposition processing on radiation images have heretofore been disclosed in, for example, U.S. Pat. No. 4,356,398. In general, radiation images are used for diagnoses of illnesses and for other purposes. When a radiation image is used for such purposes, it is required that even small differences in the radiation energy absorption characteristics among structures of an object can be detected accurately in the radiation image. The extent, to which such differences in the radiation energy absorption characteristics can be detected in a radiation image, is referred to as the contrast detection performance or simply as the detection performance. A radiation image having better detection performance has better image quality and can serve as a more effective tool in, particularly, the efficient and accurate diagnosis of an illness. Therefore, in order for the image quality to be improved, it is desirable that the detection performance of the radiation image be improved. The detection performance is adversely affected by various noises.
Superposition processing is carried out in order to reduce the aforesaid noises markedly so that even small differences in the radiation energy absorption characteristics among structures of an object can be found accurately in a visible radiation image, which is reproduced finally, i.e. the detection performance of the radiation image can be improved markedly. Specifically, a radiation image is stored on each of a plurality of stimulable phosphor sheets, which have been placed one upon another. Thereafter, an image read-out operation is carried out for each of the stimulable phosphor sheets. A plurality of image signals, which have been obtained from the image read-out operations, are added to one another. In this manner, various noises described above can be reduced.
By way of example, when superposition processing is to be carried out, two stimulable phosphor sheets have heretofore been housed in a cassette such that they may overlap one upon the other. Radiation images of an object are then recorded on the two stimulable phosphor sheets housed in the cassette. Thereafter, an image read-out operation is carried out on each of the two stimulable phosphor sheets, and two image signals are thereby obtained. The two image signals are then added to each other.
Also, techniques for carrying out subtraction processing on radiation images have heretofore been known. When subtraction processing is to be carried out, two radiation images recorded under different conditions are photoelectrically read out, and digital image signals which represent the radiation images are thereby obtained. The image signal components of the digital image signals, which represent corresponding picture elements in the radiation images, are then subtracted from each other, and a difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images. With the subtraction processing method, two digital image signals are subtracted from each other in order to obtain a difference signal, and the radiation image of a specific structure can be reproduced from the difference signal.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order to extract the image of a specific structure of an object from the image of the entire object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure of the object is enhanced by the injection of contrast media. In the latter method, an object is exposed to several kinds of radiation having different energy distributions. Alternatively, the energy distribution of the radiation carrying image information of an object, is changed after it has been irradiated onto one of at least two radiation image recording media, after which the radiation impinges upon the second radiation image recording medium. In this manner, at least two radiation images, in which different images of a specific structure of the object are embedded, are obtained. Thereafter, the image signals representing at least two radiation images are weighted appropriately, when necessary, and subjected to a subtraction process, and the image of the specific structure of the object is thereby extracted.
Subtraction processing is extremely effective, particularly for medical diagnosis, and electronics research has continued to develop improved subtraction processing methods.
However, the problems described below are encountered in the superposition processing and the subtraction processing of radiation images, wherein stimulable phosphor sheets are utilized.
Specifically, when each of the superposition processing method and the subtraction processing method utilizing the stimulable phosphor sheets is to be carried out, at least two stimulable phosphor sheets are inserted into an image recording apparatus one after the other or simultaneously, and radiation images to be subjected to the superposition processing or the subtraction processing are recorded on the stimulable phosphor sheets. Thereafter, each of the stimulable phosphor sheets is inserted into an image read-out apparatus and exposed to stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by each stimulable phosphor sheet is detected, and the radiation image stored on the stimulable phosphor sheet is thereby read out. In such cases, even if the operations for recording and reading out the radiation images are carried out very carefully, a shift and a rotation will occur between the images to be subjected to the superposition processing or the subtraction processing. As a result, in the superposition processing, even if various noises are averaged and reduced, the entire area of the superposition image, which is obtained from the superposition processing, particularly edges of a structure in the superposition image, will become unsharp. Therefore, a superposition image cannot be obtained which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. Also, in the subtraction processing, as a result of the shift and the rotation occurring between the images to be subjected to the subtraction processing, an image pattern to be erased in a subtraction image, which is obtained from the subtraction processing, cannot be erased. Alternatively, an image pattern to be formed in the subtraction image will be erased, and an artifact will occur. Therefore, an accurate subtraction image cannot be obtained. In this manner, the shift and the rotation occurring between the images to be subjected to the superposition processing or the subtraction processing adversely affect the image quality of the image obtained from the superposition processing or the subtraction processing.
The radiation image is stored as a latent image on the stimulable phosphor sheet and cannot be viewed directly like an X-ray image recorded as a visible image on X-ray photographic film. Therefore, the positions of two or more radiation images stored on the stimulable phosphor sheets cannot be visually matched to each other. Accordingly, if the shift and the rotation occur between the radiation images stored on the stimulable phosphor sheets, the shift and the rotation cannot be eliminated easily.
Also, even if the shift and the rotation between two radiation images can be detected by some means, considerable time will be required for conventional operations to be carried out in order to correct the image signals detected from the radiation images, particularly in order to eliminate the rotation between the radiation images. This is a very real problem in practical use.
In U.S. Pat. No. 4,710,875, the applicant proposed a subtraction processing method for radiation images, wherein a marker having a shape such that it may provide a reference point or a reference line is utilized. With the proposed method, image patterns of the marker are recorded on two stimulable phosphor sheets such that the patterns of the marker may be located at positions fixed with respect to radiation images stored on the stimulable phosphor sheets. When the radiation images are read out from the stimulable phosphor sheets, the patterns of the marker are detected. The amounts of a shift and a rotation between the two radiation images are then calculated with reference to the patterns of the marker. Thereafter, either one of the radiation images to be subjected to subtraction processing is digitally rotated and/or translated in accordance with the calculated amount of the rotation and/or the calculated amount of the shift. The image signal components of the image signals, which represent corresponding picture elements in the radiation images, are then subtracted from each other. The position adjusting step, which is carried out in the subtraction processing method for radiation images utilizing the marker, can also be applied to the aforesaid superposition processing method. In such cases, after the positions of the radiation images are digitally matched to each other, the image signal components of the image signals, which represent corresponding picture elements in the radiation images, may be added to each other.
However, with the proposed method, each time a radiation image of an object is recorded on a stimulable phosphor sheet, the pattern of the marker must be recorded together with the object image on the stimulable phosphor sheet. Also, the problems occur in that the image information of the object cannot be obtained from the portion of the radiation image stored on the stimulable phosphor sheet, which portion overlaps upon the position of the pattern of the marker.